1. Field of the Invention
The present invention relates to a control device for an internal combustion engine, for controlling the amount of internal EGR gas that is a burnt gas remaining in a cylinder.
2. Description of the Related Art
In a related-art spark-ignition internal combustion engine using gasoline as a fuel, a thermal efficiency is high in an operating region in which an internal combustion engine load is middle to high. On the other hand, the thermal efficiency is low in an operating region in which the internal combustion engine load is relatively low. The operating region with the relatively low internal combustion engine load is frequently used for automobiles. As a factor of reduction in thermal efficiency under the low load condition described above, the following is given. Specifically, a throttle valve is closed to reduce an air amount to be taken into a cylinder. As a result, negative work (that is, a pumping loss) increases or an effective compression ratio is lowered due to reduction in air amount itself.
Reduction in pumping loss and improvement of the effective compression ratio can be achieved by opening the throttle valve to increase an amount of intake-air. In a case where gasoline is used as the fuel, however, an air-fuel mixture obtained by mixing gasoline with air of a certain amount or larger with respect to an amount of the gasoline does not burn. Therefore, the amount of intake-air cannot be greatly increased. Thus, a great effect of improving the thermal efficiency is not obtained. However, when a temperature of intake air is high, a maximum air amount in the combustible air-fuel mixture increases. Therefore, the improvement of the thermal efficiency can be expected. However, means for heating the intake air is required. Thus, a device configuration becomes complex.
Therefore, as means for reducing the pumping loss and improving the effective compression ratio, a technology of leaving a combustion gas in the cylinder in a state in which the amount of intake-air is maintained is used. Although the thus left combustion gas scarcely contains oxygen, oxygen that is necessary for the combustion of gasoline is ensured in the intake air. Therefore, the air-fuel mixture can be combusted by the combustion gas at high temperature, which is left in the cylinder. Leaving the combustion gas in the cylinder is hereinafter referred to as “internal EGR”, and the combustion gas left in the cylinder is hereinafter referred to as “internal EGR gas”.
Further, a total gas amount of the air-fuel mixture is increased by the addition of the combustion gas to gasoline and air due to the internal EGR. Therefore, the pumping loss can be reduced, while the effective compression ratio can be improved. As a result, even if the internal combustion engine operates under the low load condition, a high thermal efficiency can be achieved. Further, a temperature of the internal EGR gas is high. Therefore, a part of thermal energy, which is hitherto disposed of as an exhaust gas, is recovered in a subsequent combustion cycle. The recovery of thermal energy also contributes to the improvement of the thermal efficiency.
In order to realize the internal EGR that provides the effects of improving the thermal efficiency as described above, advancing a valve-closing timing of an exhaust valve is effective. In some cases, however, the temperature of the air-fuel mixture at the time of compression changes to cause pre-ignition or misfire depending on a difference of the temperature of the combustion gas and the temperature and amount of the intake air. Therefore, in order to prevent both the pre-ignition and the misfire from occurring, the internal EGR gas amount is required to be adjusted properly by controlling the valve-closing timing of the exhaust valve in accordance with an operating state.
Therefore, as means for adjusting the internal EGR gas amount as described above, there has been proposed, for example, a technology of controlling the valve-closing timing of the exhaust valve to a preset timing in accordance with the operating state of the internal combustion engine (see, for example, Japanese Patent Application Laid-open No. 2009-150288). Further, in the related art described in Japanese Patent Application Laid-open No. 2009-150288, data of the valve-closing timing of the exhaust valve, which is prepared in advance for each combination of a shaft rotation speed of the internal combustion engine and the internal combustion engine load (hereinafter referred to as “combustion load”), is used for the control. In addition, control for correcting the valve-closing timing in accordance with a cooling-water temperature for the internal combustion engine is also performed.
However, the related art has the following problems.
In the related art described in Japanese Patent Application Laid-open No. 2009-150288, the valve-closing timing of the exhaust valve is controlled by referring to a database that is acquired in advance under conditions where the shaft rotation speed and the combustion load are kept constant. Specifically, under the operating condition with one combination of the shaft rotation speed and the combustion load, the valve-closing timing of the exhaust valve at which the internal EGR gas amount becomes optimum in a case where the combustion gas having the stabilized temperature under the operating condition is left in the cylinder is prepared as the database.
Here, the internal EGR gas actually remaining in the cylinder in the current cycle corresponds to the combustion gas that is derived in the combustion in the same cylinder in the previous cycle, specifically, one cycle before the current cycle, and remains in the cylinder. Thus, in a process in which the shaft rotation speed or the combustion load in the previous cycle changes in the current cycle, a difference is derived between the internal EGR gas temperature that is taken into consideration in the database so as to correspond to the shaft rotation speed and the combustion load in the current cycle and the internal EGR gas temperature actually remaining in the cylinder in the current cycle (specifically, an exhaust gas temperature of the combustion gas derived in the previous cycle).
As the difference described above, for example, in a case where the temperature of the internal EGR gas actually remaining in the cylinder in the current cycle is higher than the internal EGR gas temperature that is taken into consideration in the database so as to correspond to the shaft rotation speed and the combustion load in the current cycle, the temperature of the air-fuel mixture at the time of compression becomes higher. Thus, there is a problem in that knocking due to pre-ignition occurs.
On the other hand, in a case where the temperature of the internal EGR gas actually remaining in the cylinder in the current cycle is lower than the internal EGR gas temperature that is taken into consideration in the database so as to correspond to the shaft rotation speed and the combustion load in the current cycle, the temperature of the air-fuel mixture at the time of compression becomes lower. Thus, there is a problem in that misfire occurs.